The present invention relates to short transmission distance optical communication systems and, more particularly, to such systems based on using a portion of a visual display as the transmitter in such a system.
Television receivers and computers have become ubiquitous, and are found in homes, offices, etc. Both the television receiver and the display monitor used in connection with computers provide visual displays for the benefit of users. Such users typically have a substantial capability to control what is displayed on the screen of a computer display monitor to the point of interacting with that display to control modifications thereof. Television users, on the other hand, are usually restricted insofar as the display presented on the screen of a television receiver to just that which is shown as the result of a channel selection or of a video tape selected for use in a video tape recorder connected to supply video signals to the television receiver.
Nationally determined broadcasting standards and economics have limited the opportunities to provide significant numbers of television receivers with broader capabilities. There have, however, been some added capabilities provided to some television receivers insofar as adding supplemental signals for transmission along with the usual video display control signals to thereby provide additional information to viewers or users which can be used for various purposes. One such system is the transmission of digital information in these supplemental signals provided during blanking periods in the usually transmitted video display control signals that control the displays used in presenting ordinary programming on the screen of the television receiver. Such signals have been used to provide television schedules, stock quotes, etc. The system can be interactive if the television receiver is connected to a two-way cable system, the telephone system, or in some other two-way transmission arrangements. However, such systems are often impractical for home use because of cost, scheduling limits, technical complexity and the like.
A further method of transmitting supplementary signals which ameliorates some of these difficulties is one using a portion of the display screen of a television receiver as an optical signal transmitter. Video display control signals from a television camera, or from a video tape recorder, or from some other source controlling the visual images displayed on the screen of a television receiver picture tube are used in the conventional manner to provide the primary visual images on that display screen in all but a selected small portion thereof. In this selected portion, however, a digital optical signal is generated so as to convert that portion of the display screen into serving as a transmitter of such an optical signal. The receiver of this optical signal comprises an optical signal processing system, including a photodetector mountable across from this optical transmitter portion of the display screen to effectively form an optical signal channel therebetween. Once the optical signal generated by the optical transmitter portion of the display screen is through the channel to impinge on the photodetector, that photodetector converts the optical signal to a corresponding electrical signal which can be used as appropriate in the remainder of the signal processor typically in connection with the visual images provided on the television display screen.
The baseband optical signal provided at the transmitter portion of the television receiver display screen is baseband modulated in some manner to carry with it the digital information intended to be transmitted from a data source or sources, controlling the formation of the video display control signals in the television receiver, to the remainder of the signal processor connected to the photodetector that is to make use of such information. Once such modulation technique known is to have a relatively large intensity light level in the transmitter display screen portion represent one logic state in a binary logic system, and have a relatively small light intensity in that portion represent the remaining logic level state in such a system. Other known possibilities are varying the amplitude of the light in the transmitting location between other, different levels of intensity or color to represent binary logic states, having more than one screen display transmitting portion so that the different positions among them exhibiting a substantial light intensity could represent these logic states, or some variations in frequency of the occurrence of substantial intensity pulses could be used for this purpose. However, each of these methods is susceptible to the introduction of transmitted data error because of the nature of the environment in which the optical signal is generated and the deficiencies in television signals and screen displays.
A first concern is the reflection of light, originating externally to the transmitter display screen portion, being directed into that portion to reflect therefrom or from the cabinet window surfaces into the photodetector. Another concern is the occurrence of "ghost" images on the television display screen offset somewhat from one another which arise because of the television signals propagating over multiple paths to the television receiving antenna. In addition, images formed on the television display screen suffer from position jitter as well as from noise arising from a variety of sources. Further, television receivers age so that a wide range of television tube conditions can be encountered over a variety of television receivers.
Such occurrences and deficiencies in the electromagnetic radiation reaching the photodetector often lead to corresponding erroneous electrical output signals from that photodetector. These output signals are then applied to a magnitude based signal detector that correspondingly detects these occurrences as erroneous digital data values. Many of the methods that might otherwise be suitable to reduce the problems leading to these errors are costly, complicated or bulky or all of these. Thus, there is desired a system for transmitting information from a portion of a visual display over an optical channel to a detecting device which more reliably acquires the data intended to be sent while using a relatively inexpensive system.